Whilst browsing the web earlier today I found this easy to read article, with definitions of the most popular video codecs.

Decoding The CodecsA codec is a formula that removes data - redundant data, data you aren't likely to notice, data that can be derived from other data, and so on - so files can be transmitted more quickly. At the receiving end, a decoder reassembles the data into something coherent.

The Wired article goes into detail about various popular Codec formats including: